This presentation was given at the Oosting Memorial Symposium on April 17, 2009. The authors were Yi-Hsin Erica Tsai and Paul S. Manos. Abstract: Because species respond individually to climate change, understanding community assembly requires examination of multiple species from a diversity of forest niches. We present the phylogeographic history of an understory, parasitic herb (Epifagus virginiana, beechdrop) that has an obligate and host specific relationship with a common eastern North American forest tree (Fagus grandifolia, American beech). The migration histories of the host and parasite were compared to elucidate potential limits on the parasite’s range and to understand their responses to shared climate change. Two chloroplast DNA regions were sequenced and landscape genetic and coalescent methods were used to reconstruct the post-glacial migration history of the parasite. Epifagus virginiana is shown to have migrated from the southern Appalachians into the Northeast and then westward into the Midwest. The parasite's pattern of expansion parallels the development of beech forests but differs from the routes of initial range expansion of beech. This suggests that host density effects drive the distributional changes in the parasite, which are further confirmed in fine scale studies. The composite migration history of this parasite and its host shows how two diverse components of a forest community colonized the landscape separately -- even as aspects of the host's distribution greatly influenced the path of the parasite -- before reassembling into their present day co-distributed range. The animations from the powerpoint were partly converted using code from Neil Mitchell's Haskell Blog, http://neilmitchell.blogspot.com/2007/11/creating-pdf-from-powerpoint-with.html.